Science a GoGo's Home Page Forums General Discussion Physics Forum Pesky gravity waves

Haha sorry Bill I really was having a joke with you

You aren't getting it and so lets see if we can explain the joke.

Remember your claim as per your example above that you can practically show me a gravity wave

The problem is the stress-energy tensor for a gravitational field has to be set to zero ... you can't calculate it hence why there is no calculation for it

Hopefully you get my little jibe at you now, I thought you would/should know that fact

Here is how uni students usually run across the problem and read Peter Donis the staff moderators very careful answer

https://www.physicsforums.com/threads/stress-energy-tensor-for-gravitational-field.619006/

The 2 points you need to take home

1. The stress-energy tensor on the RHS of this equation has *no* contribution from the gravitational field. The reason this is important is that the Einstein tensor on the LHS of this equation obeys the Bianchi identities, i.e., its covariant divergence is identically zero; therefore, the covariant divergence of the RHS is also identically zero. This is how local conservation of energy is expressed in GR, and it *only* works with the equation written the way I wrote it, with no contribution to the stress-energy tensor from gravity.

2.The various pseudo-tensors describing "energy in the gravitational field" are derived by extracting some piece of the LHS of the above equation, moving it to the RHS, and calling it tμν. But there is no one unique way to do that, because there is no one unique way to split up the true Einstein tensor into a "background" part that stays on the LHS, and an "energy in the field" part that goes to the RHS. You describe two possible ways of doing it, taking either the Minkowski metric as the "background" or some more general curved metric (such as the Schwarzschild metric) as the "background" on which small-scale perturbations (such as gravitational waves) are superimposed. There are others. Which method you use depends on what you are trying to do with it; there is no one "right" answer.

In the original link I gave you they distilled this down to a much simpler semi-layman terminology.


So all you need to do now is convince me you can seperate your wave from the background metric in your example .... hopefully you get the joke

I understood what you were sort of trying to do with your layman example but at a technical level it goes absolutely nowhere.